Much of the population growth will occur in so-called “high fertility” countries — where each woman is having, on average, more than 1.5 daughters — in Africa, Asia, Oceania and Latin America, according to the report. While populations in low- and intermediate-fertility nations are expected to peak before the end of the century, the population in high-fertility nations will continue to increase.

In Africa, where growth already threatens to overwhelm over-stretched food and water resources, the population could more than triple, from about 1 billion today to more than 3.6 billion. World population is expected to pass 7 billion later this year.

The report, prepared by the UN’s Population Division, projects that there will be 9.3 billion people by mid-century, which is 156 million more than the group predicted in a 2008 report. Projections have increased because fertility has not declined as rapidly as expected in poorer countries and has increased slightly in wealthier nations.